1. Technical Field
The invention concerns a method for producing a lengthwise welded metal tube with an outside diameter of 1 to 6 mm, whereby a metal band is drawn from a storage reel and is shaped into a lengthwise slotted tube by a forming tool, and the lengthwise slot is welded by a laser welding installation.
2. Description of the Prior Art
The manufacture of metal tubes in the indicated range of diameters from a metal band, which is shaped into a slotted tube and whose lengthwise slot is welded by a laser, is known from U.S. Pat. No. 4,759,487. The metal band, e.g. a band of stainless steel, is gradually shaped into a lengthwise slotted tube by a first forming tool, which contains several pairs of shaping rollers. A second forming tool, which also contains several pairs of shaping rollers, transforms the tube with the lengthwise slot into a tube with abutting band edges. The shaping rollers of the second forming tool only touch the tube on the outside surface. A pair of rollers is provided downstream of the second forming tool and guides the slotted tube at a predetermined distance from the focal point of a laser welding installation. After the welding installation, the tube enters a cooling tube containing a large clearance, in which the welded seam is thoroughly cooled with argon. The welded metal tube then enters a drawing device which reduces its diameter. The puller which draws the metal band from the storage reel and pulls it through the shaping tools and the drawing device, is a motorized take-up reel around which the metal tube is wound 180.degree.. The take-up reel has a V-groove around its periphery, into which the metal tube is pressed so that a sufficient pulling force is exerted. However, this can produce an oval deformation of the metal tube. The lateral orientation of the lengthwise slot with respect to the laser beam takes place through the top rollers of the second forming tool, which contain peripherally extending protrusions that dig into the slotted tube and guide the band edges.
In addition to the danger of deforming the finished metal tube in the V-groove of the take-up reel, it is a disadvantage that this method cannot provide an accurate alignment of the lengthwise seam with respect to the laser beam. The focal point of the laser beam is oftentimes located above the lengthwise slot creating a large "focal spot" covering the lengthwise seam, which requires greater laser energy or leads to a lower production speed.